(a) Field of the Invention
This invention relates to a circuit arrangement for monitoring the faulty current distribution (current imbalance) in two converter branches, connected in parallel, each of which contains at least two series-connected converter valves. More particularly, a differential current transformer measures the difference between the currents flowing at the junction point of the two converter branches; the anodes and cathodes, respectively, of adjacent converter valves in both converter branches are connected to each other by cross-connections each of which contain the primary winding of a current transformer; and threshold detectors are associated with each current transformer for initiating protective measures.
(b) Description of the Prior Art
In converter installations of high power rating which serve, for instance, as part of a high voltage DC transmission system, converter valves connected in parallel are used because the current carrying capacity of individual available converter valves is limited. Such arrangements are built up from several series-connected converter valves in order to overcome the limited reverse voltage rating of a single converter valve. To avoid overloading individual converter branches, the current in the converter branches must be monitored so that uniform division occurs.
German Auslegeschrift 25 21 262, at FIG. 4 shows a circuit arrangement of the type just mentioned in which the anodes and cathodes, respectively, of adjacent converter valves in the parallel converter branches are connected to each other via cross connections each containing the primary winding of a current transformer. A differential current transformer is also provided at a junction point of the two converter branches which measures the difference of the currents flowing into the junction point in the two converter branches. The output voltage of the differential current transformer and the current transformers taken off at a load is fed to a respective threshold detector which responds and activates a refiring device, addressing firing pulses to the control path of the valve carrying the smaller partial current. If, therefore, the transverse current in a cross-connection exceeds a predetermined amount, a converter valve is refired. A considerable transverse current can occur when one of the valves of the converter branches does not become conducting upon a first firing pulse.
In such a circuit arrangement, the current in the individual converter valves is not determined exactly; conclusions are merely drawn from an increase in the transverse current that a converter valve lacks conductivity. In a parallel circuit having a multiplicity of series connected converter branches, however, rectified currents can flow in the cross connections whose magnitude remains below the response threshold of the threshold element and which add up, toward the end of the converter branch, to a toal current leading to an overload and possible destruction of the converter valves near the end. The trouble just described cannot be detected by the circuit of German Auglegeschrift 25 21 262.
To provide for this kind of trouble, it would seem necessary to measure the current flowing in each converter valve by means of a separate current transformer. This, however, necessitates a large expenditure for current transformers and results in an increase of the resistance of the converter branches from insertion of the current transformers in the series branches. Inserting current transformers into the series branches also involves considerable costs because the electrical connections between the individual branch converter valves, which may already be prefabricated, must be opened up.
It is an object of the invention to detect the faulty current distribution in adjacent converter valves of parallel connected converter branches without the need to assign a separate current transformer to each converter valve in the series branch.
Faulty current distribution is herein understood to mean the departure of the level of current flowing in a converter valve from one half the value of the total current flowing at the junction point of the two converter branches.